1. Technical Field
The present invention relates to a pressure sensor and a method for manufacturing the same. This pressure sensor does not use oil as a pressure receiving media and therefore is suitable for broadening a scope of application of the pressure sensor.
2. Related Art
Pressure sensors that include piezoelectric resonating elements as pressure sensing elements are known as water pressure meters, barometers, and differential manometers. Piezoelectric resonating elements include, for instance, electrode patterns on planar piezoelectric substrates, having a detection axis in a direction in which a force is detected. When a pressure is applied in the direction of the detection axis, a resonant frequency of the piezoelectric resonating element changes and the pressure is detected using the fluctuation in the resonant frequency. Pressure sensors that use piezoelectric resonating elements as pressure sensing elements are disclosed in JP-A-56-119519, JP-A-64-9331, and JP-A-2 -228534. When a pressure is applied to a bellows from a pressure inlet, a force F corresponding to an effective area of the bellows is transmitted to the piezoelectric resonating element as either a compressive force or a tensile force through a force transmitting means that has a pivot as a fulcrum (bend hinge). A stress due to this force F occurs in the piezoelectric resonating element, and this stress changes the resonating frequency. The pressure sensor measures pressures by detecting changes in the resonant frequencies of the piezoelectric resonating element.
The pressure sensor according to related art will be described with reference to examples disclosed in JP-A-56-119519, JP-A-64-9331, and JP-A-2-228534. FIG. 8 is a sectional drawing illustrating a structure of a pressure sensor according to these examples of related art.
A pressure sensor 101 according to related art shown in FIG. 8 includes a case 104 having a first and a second pressure input orifices 102 and 103 that are arranged to face each other, and a force transmitting member 105 arranged inside the case 104. A first end of the force transmitting member 105 is sandwiched with and coupled to one end of a first bellows 106 and one end of a second bellows 107. The other end of the first bellows 106 is coupled to the first pressure input orifice 102, and the other end of the second bellows 107 is coupled to the second pressure input orifice 103. Moreover, a tuning fork resonator 109 is arranged between a second end of the force transmitting member 105 and an end of a substrate 108, at a side opposite from a pivot (fulcrum).
Fluid is filled inside the bellows of this pressure sensor for detecting pressures in a high precision. Generally, oils such as silicon oil which has high viscosity are used as the fluid, in order to avoid bubbles entering and accumulating inside the bellows or between the folds of the bellows.
Viscous oil 110 is filled into the interior of the first bellows 106 that receives a pressure of the fluid subjected to measuring. In the case of measuring fluid pressure, the oil 110 contacts and faces the fluid at an opening 111 opened at the first pressure input orifice 102. The size of the opening 111 is set so that the oil 110 does not leak out.
In such a pressure sensor 101, when the fluid subjected to pressure measurement applies the pressure F to the oil 110 which is filled inside the first bellows 106, this pressure F is then applied to the first end of the force transmitting member 105 (the pivotably supported swing arm) through the first bellows 106. At the same time, the atmospheric pressure is applied to the second bellows 107 and a force equivalent to the atmospheric pressure is applied to the first end of the force transmitting member 105.
Consequently, a force equivalent to a differential pressure is applied to the double-ended tuning fork resonator 109 through the second end of the force transmitting member 105, either as a compressive force or a tensile force with a pivot of the substrate 108 as a pivoting point. Here, the differential pressure means a pressure difference between the atmospheric pressure and the pressure F applied by the fluid subjected to pressure measurement. One of the compressive force and the tensile force applied to the double-ended tuning fork resonator 109 changes a resonant frequency of the double-ended tuning fork resonator 109 in accordance with a strength of the stress. Therefore, measuring this resonant frequency allows for detecting the size of the pressure F.
Another pressure sensor that uses diaphragm as means to receive pressure that is subjected to measuring is disclosed in JP-A-2003-42896. As shown in FIG. 9, this pressure sensor includes a diaphragm 201 formed in a shape of either a disk or a plate, and a semiconductor (piezoresistive element) 202 is formed subsequently to forming a silicon oxide thin film and a chromium oxide thin film on the surface of the diaphragm 201. If the diaphragm 201 bends when receiving pressure, the semiconductor 202 distorts, causing a fluctuation in the ohmic value of the semiconductor 202. The pressure sensor gets this ohmic value fluctuation as an electric signal, thereby detecting the pressure change.
JP-A-2006-194736 and JP-A-2007-132697 disclose pressure sensors which are fixed to engine blocks so as to be used to detect hydraulic pressures inside the engines. Such pressure sensors are shown in FIGS. 10 and 11. A pressure sensor 301 includes a sensing unit 302 that outputs electric signals which correspond to an applied pressure, a pressure-receiving diaphragm unit that receives pressure, and a pressure transmitting member 304 for transmitting the pressure from the pressure-receiving diaphragm to the sensing unit 302. Specifically, a first diaphragm 306 and a second diaphragm 307 are respectively installed on each end surface of a hollow metal stem 305, the first diaphragm 306 receiving pressure, and the second diaphragm 307 detecting pressure. The pressure transmitting member 304 is provided between the first diaphragm 306 and the second diaphragm 307 of the stem. The pressure transmitting member 304 is a shaft made of either metal or ceramic, and is provided between a pair of diaphragms (first diaphragm 306 and the second diaphragm 307) in a prestressed state. Further, a chip with a strain gauge functionality (the sensing unit 302) is installed to an outer end surface of the second diaphragm 307 as a pressure detection element, and the pressure transmitting member 304 transmits a pressure received by the first diaphragm 306 to the second diaphragm 307, so that the deformation of the second diaphragm 307 is converted to electronic signals by the strain gauge chip (the sensing unit 302), thereby detecting the hydraulic pressure of the engine.
As recited in JP-A-56-119519, JP-A-64-9331, and JP-A-2-228534, according to the pressure sensor shown in FIG. 8, the oil 110 filled into the first bellows 106 has higher thermal expansion coefficients compared to other elements that constitute the pressure sensor 101, such as the force transmitting member 105 and the double-ended tuning fork resonator 109. Therefore, a temperature change causes thermal distortion of the components constituting the pressure sensor. Such thermal distortion works on the double-ended tuning fork resonator 109 as an unwanted stress, resulting in measured pressure value errors, thereby degrading the characteristics of the pressure sensor.
Moreover, since the oil 110 filled in the first bellows 106 contacts and faces the fluid subjected to pressure measuring, the oil 110 may flow out into this fluid, or, the fluid may flow into the first bellows 106 depending on how the pressure sensor is installed. This may generate bubbles inside the oil 110 filled inside the first bellows 106. A force cannot be stably transmitted through the force transmitting member 105 to the double-ended tuning fork resonator 109, if bubbles are generated in the oil 110 which serves as a pressure transmitting media, thereby possibly inducing an error in the measured pressure value.
Moreover, as described, since the oil 110 contacts and faces the fluid subjected to pressure measuring, the oil 110 may flow out into this fluid depending on how the pressure sensor is installed. Therefore, the pressure sensor using the oil 110 according to related art cannot be used for measuring the pressure of pure fluid that disfavors foreign substance.
In the technique described in JP-A-2003-42896, a semiconductor is formed on a surface of the diaphragm, and therefore the cost of the diaphragm significantly increases. Moreover, in the case of differential pressure detection in which diaphragms are respectively arranged for a pressure receiving unit as well as for a sensing unit, the behavior of the diaphragms differs between the pressure receiving side and the sensing unit side. This is due to the state where while the diaphragms have the same shape, the chip is formed on one of them and thus the mechanical deviations of the diaphragms are not identical. Consequently, there is a drawback of not being able to improve the measurement precision.
According to JP-A-2006-194736 and JP-A-2007-132697, a diaphragm and a shaft are in contact with each other under load. Since a pressure sensor is used at a high temperature in a high pressure, if the diaphragm and the shaft were rigidly fixed, this mechanism may be damaged by the difference in thermal expansion between the components. For this reason, the diaphragm and the shaft only have a point contact, and are not bonded by bonding means such as adhesives. Therefore, there is a very high possibility that this point contact deviates when the pressure change operates the diaphragm and the shaft. As the point contact deviates, a force working in both the diaphragm and the shaft leaks out, resulting in pressure detection with insufficient precision. Moreover, the pressure sensors described in JP-A-2006-194736 and JP-A-2007-132697 are used at a high temperature in a high pressure so as to detect the pressure inside the engine combustion chamber. Therefore, it is desirable that the force transmitting member be as long as possible in order to create a distance between the pressure receiving unit and the sensing unit, thereby avoiding thermal effect on the components such as the chip of the sensing unit. This has not been suitable for application in the techniques which require size reduction. In addition, in the case of JP-A-2006-194736 and JP-A-2007-132697, a force is transmitted having a shaft between a pair of diaphragms. However, since a sensor chip is directly attached to the diaphragm on the sensing unit side, the property of the diaphragms differs between the pressure receiving side and the sensing unit side. While the diaphragms have the same shape, the chip is formed on one of the diaphragms and thus the mechanical deviations of the diaphragms are not identical, resulting in a drawback of not being able to improve the measurement precision.